The present invention relates to a method for making fumed silica and product obtained therefrom. More particularly, the present invention relates to burning a mixture of a fuel, such as hydrogen, a silicon compound, such as a silane or an organosilane, and oxygen or an oxygen containing gas in a combustion chamber to produce aggregates having an average convex perimeter in the range of about 0.12 micron to about 0.60 micron.
Prior to the present invention, various procedures were used to prepare finely divided silicon dioxide by burning a mixture of an organohalosilane, hydrogen and an oxygen containing mixture. Some of these methods are shown by Lange et al, U.S. Pat. 3,954,945, Lee, U.S. Pat. No. 4,048,290 and Kratel et al, U.S. Pat. No. 4,108,964.
Existing methods for making finely divided metal oxides, such as hydrophilic silicon dioxide are generally based on a procedure involving feeding a combustible mixture of a metal or metalloid halide, hydrogen, and an oxygen-containing gas such as air, to a burner in a combustion chamber, which results in the production of primary metal oxide particles, such as particles of silicon dioxide. The primary metal oxide particles form aggregates having an average convex perimeter of about 0.5 micron to 1.5 microns which can often exceed the optimum average aggregate size desired in particular applications. The term "convex perimeter" is defined hereinafter as the average circumferential distance from high point to high point around the aggregate measured from TEM micrographs.
A method for reducing aggregate size is shown by Kratel et al, U.S. Pat. No. 3,953,487. The Kratel et al process involves reducing aggregate size by subjecting hydrophobic silicon dioxide aggregates composed of interlaced chains of primary SiO.sub.2 particles to a mechanical impact treatment. Size reduction is used in combination with water and in further combination with an organosilicon compound, such as hexamethyldisilazane. The reduced hydrophobic silicon dioxide aggregates have been found to have a smaller average convex perimeter. It would be desirable to provide alternate methods for directly making fumed silicon dioxide aggregates which did not require an additional milling step.